384 



EMBRYOLOGY 



Further Developmental Processes of Both Types. 



— Thus far we have chiefly considered the external shape of 

 the Tornaria. As regards the internal development, we find 

 confirmed the processes which, following Bateson, we have 

 already described. In the youngest stages of Tornaria yet 

 observed (Fig. 170), the archenteron, which in this case 

 never loses its connection with the ectoderm, develops an 

 unpaired evagination. This is said to be the fundament of 

 the so-called water-vascular vesicle, which, like the corre- 

 sponding organ of the Echinoderm larva, opens out by means 

 of a pore on the dorsal surface (Fig. 169 A). This in 

 particular has given rise to a comparison with the Echino- 

 derm larvae. In addition to this diverticulum, two pairs of 

 evaginations arise farther back on the intestine (Agassiz). 

 They are the coelomic sacs, which soon 

 become detached from the intestine, 

 and lie close to it on either side as 

 two pairs of flattened vesicles. They 

 soon become considerably enlarged, 

 and then their walls are applied to the 

 wall of the body and that of the in- 

 testine as the somatic and splanchnic 

 layers.^ A mesentery is developed 

 dorsally and ventrally, separating the 

 sacs of the body cavity of the two 

 sides ; but, according to Spengel, the 

 dorsal mesentery may afterwards de- 

 generate. The hinderraost pair of 

 coelomic sacs supplies the greater 

 part of the body cavity, — namely, that of the entire trunk, 

 — whei'eas the body cavity of the collar arises from the 

 anterior pair, and the cavity of the acorn is developed from 

 the so-called water-vascular vesicle (Spengel). The re- 

 semblance of the latter to the paired structures points to 

 the fact that they originally had the same significance, and 

 the appearance of two acorn pores in Balanoglossus Kupfferi 



1 As is to be seen in Fig. 172 (p. 387), entirely similar conditions 

 appear also in Balanoglossus Kowulei skii, which does not develop by 

 means of a Tornaria. 



cr/i 



Pig. 170.— Early stage of 

 a Tornaria (after Goette, 

 from Balfoüe's Compara- 

 tive Emhryology). m, mouth ; 

 an, anus ; W, so-called 

 water-vascular vesicle. 



